Let us begin with a tragic mystery. Sometimes an ominous
saucer shaped cloud is seen to descend from a storm cloud as shown below.

Top: Beneath the updraft of a thunderstorm , a "collar
cloud" or "wall cloud" forms which appears as an upside-down
stump at Tuscaloosa, Alabama. Bottom: A tornado is visible underneath this
very large wall cloud, which will kill three people in Linden, Tennessee.
Photo courtesy of Curtis Williams at New Albany, Mississippi.

Spotters recognize this feature as a signal that an intense
updraft is in this part of the storm, and watch it for the development
of a tornado. But how can an upwardly moving air be manifest as a lowering
of a cloud?

The process of cloud formation is absent from physics
textbooks, yet some of the most fundamental discoveries in physics came
about through its study. Although early apparatuses for producing clouds
were relatively elaborate, you can instantly demonstrate cloud formation
to a sizable audience with an item often found in the garbage. The common
polyethylene terephthalate "clear plastic" soda bottle, some
water, and a match are all that are needed. The first step is to fill a
large two or three-liter soda bottle with enough water to cover the bottom,
bringing it to 100% relative humidity. In the atmosphere cloud droplets
form around ubiquitous motes suspended in the air, and will need to be
replenished for the demonstration to be successful. In order to do this,
light the match outside and after compressing the bottle, let it take in
air from the environment, or just drop the lit match into the bottle. One
can then just blow into the bottle and release the pressure to form a cloud.
Also one can seal the bottle with a cap, and strongly grip the bottle for
a few seconds. When the pressure is released, a cloud immediately forms
inside the bottle, as shown below.

The difference between the clear and cloudy states can
be illustrated more vividly be illustrated if the bottle is placed across
an overhead projector. If the sealed bottle is compressed again, the cloud
will vanish, but is restored upon release. If the cloudy bottle is held
next to a Van de Graff electrostatic generator, it will irreversibly vanish.
The cloud may be expelled by removing the cap and compressing the bottle,
although the cloud fades immediately upon mixing with the environment.

A couple of inexpensive items from an Arbor Scientific
kit allows more careful study of the phenomenon of adiabatic heating and
cooling, one of the keys to understanding cloud formation. Currently on
the market is a fascinating device called a "Fizz Keeper". that
keeps soda from going flat. It is a piston that screws on to the neck of
a small 355 milliliter or 2 liter soda bottle. Upon pushing the piston
several times, the interior is pressurized, which inhibits bubble formation
in soda and keeps the carbon dioxide (that makes soda tart and effervescent)
dissolved for a longer time. An terrarium thermometer is a strip of liquid
crystal digits that respond to temperature within 2o F . When one is placed
inside the bottle, it responds very quickly to a change in temperature,
with a time constant of a few seconds. Such thermometers have a sticky
back and can be kept out of the liquid by sticking it to the upper interior
of the bottle. If pumped quickly enough, the interior can be heated to
the upper limit of the thermometer of 90o F. When the cap is unscrewed,
as the pressure returns to normal, the temperature plunges back to room
temperature, or even well below room temperature if the air was allowed
to cool to room temperature while pressurized. If we prepare a bottle as
before to make a cloud, one realizes that cloud formation corresponds to
the sudden lowering of temperature when the air is depressurized.

It would be fascinating to measure the pressure, temperature,
and "cloudiness" inside of the bottle for this process. This
was done as an introduction to computer-based-laboratory measurements for
a physics for education majors course, shown below.

Students in a physics-for-education-majors course learn
about computer-based-laboratory measurements with a PASCO 750 interface
measuring the pressure, temperature, and optical transmission of a cloud
formed inside a soda bottle. Right, Penni Wallace vigorously pumps the
bottle after Roger Klee has begun acquiring the data. From right to left,
Leslie Beaumont, Tameka Shamery, and Lindsey Couier monitor the data to
see if the changes they suggested in the experiment will result in a more
successful run.

Pressure, relative optical transmission, and temperature
will be monitored inside of a two-liter plastic soda bottle prepared as
described above pressurized with a Fizz-Keeper. A PASCO 750 interface will
monitor these quantities and analyze the data with the software Data Studio
® by PASCO. . Relative optical transmission was read with a PASCO CI-6504
Light Sensor outside the bottle with a flashlight mounted so that it would
shine through the bottle. The sensor consists of an optical cable that
is run into a detector box. The light-gathering ability of the cable's
head and directional quality was improved by mounting it into the body
of a one-time use camera behind the lens. The camera was then mounted opposite
to the flashlight. To make measurements inside the bottle, holes were melted
in the side by heating a nail and pressing the head against the plastic.
This allowed for the spout of an air tube to be connected to the PASCO
CI-6532 Absolute Pressure Sensor. For the PASCO CI-6525 High Accuracy temperature
probe, the bottle's cap was useful to support it. This was accomplished
by drilling a hole through the cap, and mounting it over the hole in the
bottle with the thermocouple inserted through the holes. Silicone aquarium
sealant was effective in sealing all of the connections, after realizing
it was best to put the hoses on the spouts before installing them so as
not to extirpate the sealant. One more spout was installed so that the
bottle could be slowly depressurized, by slowly unscrewing a Hoffman clamp
pinching an exit hose. The experimental setup is pictured below in greater
detail.

Pressure, temperature and optical transmission are measured
in a soda bottle with a PASCO 750 interface running PASCO Data Studio.

The students made several trials on the system to get
a successful run, and prevailed even though the bottle sprang a leak at
certain pressure (this difficulty has been eliminated with a more careful
job of sealing since). One common misconception about a true Computer-Based
Laboratory (CBL) is that one just clicks a button, and the computer will
automatically acquire a meaningful experiment. One of the particular educational
opportunities of this experiment was the very different time scales of
each of the transducers. The light sensor responds so quickly that an ac
light source cannot be used. On the other hand, the thermocouple responds
so slowly that one must perform the experiment sufficiently slowly that
it has time to "catch up" yet quickly enough so that the trial
is practically adiabatic. Issues similar to those encountered in this experiment
were thoroughly discussed in a recent article in The Physics Teacher by
Francis X. Hart in which students investigate the solidification of lard.

The top chart shows the increase in pressure as the Fizz
Keeper pumps the interior to 140 kPA. In the graph beneath it, the temperature
rises accordingly by about 1oC. Meanwhile, plenty of light gets through
with some fluctuation, probably because of jostling the bottle as it is
being pumped. The pressure is lowered by unscrewing a hose clamp, and the
temperature drops. The optical transmission drops as a cloud is formed
in the bottle. Notice that although the pressure returns to its ambient
value, the temperature remains above its ambient value. This illustrates
an important concept in weather forecasting, that for the same air mass,
it is unlikely that temperature will drop significantly below dew point,
because the water droplets releases enthaphy into the air as they form,
and is why temperatures vacillate less in more humid regions. The optical
transmission increases somewhat, perhaps because droplets begin to aggregate
and stick to the container. The bottle is pressurized again, and the opacity
(cloudiness) is even greater than before as evident by even less optical
transmission. The temperature resides at even higher ambient temperature.
As the system is pressurized a third time, the ability to make the cloud
disappear is most clearly evident, which upon release of the pressure leaves
the lowest optical transmission (thickest cloud) of all.

We can now explain why clouds form, in addition to the
answer to our mystery. As air rises the atmospheric pressure is decreased,
which cools to dew point, at which the cloud forms with the help of particles
in the air to serve as condensation nuclei. The level at which this occurs,
the lifted condensation level, corresponds to the base of a cloud. We infer
from the presence of an unusually low lifted condensation level that the
region is at unusually low pressure, corresponding to upwardly accelerating
air.

Acknowledgments

Tremendous thanks are due Craig Bohren of Penn State University,
and Charles Knight of the National Center for Atmospheric Research for
their tremendous assistance in writing this article. Thanks are also due
Gene Byrd and Pieter Vissher and Stan Jones of the University of Alabama
for their suggestions toward this article's improvement. I am grateful
to University of Alabama's Imaging Services for assistance in photographing
the article, and our dedicated library staff in finding some of the references
utilized for this article.

3. Charles Thomson Rees Wilson while working at a meteorological
observatory observed glories, a spectral optical phenomenon produced by
clouds. [An excellent discussion of glories was published recently by Edward
Pascuzzi. "The Glorious Glory." The Physics Teacher. 36. 164-166.
(March, 1998)]. To explain them he built a chamber to produce clouds. [C.T.R.
Wilson. "Condensation of Water Vapour in the Presence of Dust-Free
Air." Phil. Trans. Roy. Soc. A. 189, 265 (1897)]. Upon applying an
electric field to clear away any ions or dust that would promote condensation
, he found that the tracks of subatomic particles were made visible. The
device was essential to the discovery of the positron by Carl D. Anderson,
and the muon by S.H. Neddermeyer, and many other results. Rutherford called
the cloud chamber "the most original and wonderful (instrument) in
scientific history." (REF: Henry A. Boorse, Lloyd Motz. The World
of the Atom. (Basic Books, New York, 1966) p. 676). Another Scottish meteorologist
John Aitken developed methods of cloud condensation that were utilized
by Joseph John Thomson in some of his work on the electron.

4. The soda bottle was invented by Nathaniel Wyeth to
withhold internal pressure yet be pliable without splitting. Although a
stretched sheet of polyethylene terephthalate polymer is extremely resilient
against the direction of the polymer , it tends to split along the direction
of the polymer. Wyeth's solution was to blow the bottle from two orthogonally
crossed sheets of polymer. While his brother ___ Wyeth is an accomplished
artist, Nathaniel's "sculpture" is found on the shelves on every
store. REF: John C. Kotz, Melvin D. Joesten, James L. Wood, John W. Moore.
The Chemical World: Concepts and Applications (Harcourt Brace & Company,
Fort Worth, 1994) p. 591.

5. For a demonstration to become popular, it must be
easy to prepare from readily available materials. The famous "can
crush", in which a soda can containing boiling water is imploded by
atmospheric pressure, was motivated as a substitute for a well-known similar
demonstration with a metal fuel can, but metal fuel cans became rarer with
the use of plastic ones instead. REF: P.B. Visscher. "Simple student-repeatable
demonstration of atmospheric pressure." Am. J. of Phys. 47, 1015 (1979).

6. Aitken's paper

7. A nearly identical cloud-making procedure, accompanied
with a detailed conceptual discussion of the physical processes involved
may be found in: Craig F. Bohren. Clouds in a Glass of Beer- Simple Experiments
in Atmospheric Physics ( John Wiley & Sons, New York, 1987) pp. 8-9.

8. The brothers Adrian, Hargrove, and William "Bully"
Van de Graaff were accomplished football players for the Crimson Tide and
"Bully" was their first All-American. Robert Jemison wanted to
pursue his career in football too, but his leg injury prevented this, so
he went into physics instead. After getting his M.S. from the University
of Alabama in 1923, on a Rhodes Scholarship he developed the idea for the
famous electrostatic generator at Oxford, England and got his Ph.D. in
1928 from Queen's College. The first model generated 80,000 volts, and
a 1.5 MV version was completed at Princeton in 1931, and even more powerful
versions were completed at the Massachusetts Institute of Technology in
the years following. More than a classroom demonstration, the generator
is still used to generate high energy particles in medicine and research,
such as the radioactive ion beam facility at Oak Ridge National Laboratory
. REF: Suzanne Rau Wolfe. The University of Alabama: A Pictorial History.
(U. of Al. Press, Tuscaloosa, 1983) pp. 150.

9. Made by Jokari, U.S. Patent # 4,723,670. Available
from Arbor Scientific (cat # PI 2050) for $3.25)

10. Also available from Arbor Scientific their Fizz-Keeper
activity kit (PI-2050) in which students observe adiabatic heating and
measure the weight of air.